1. The Field of the Invention
The present invention is related generally to techniques for improving the beam quality of semiconductor lasers. More specifically, the present invention is related to a system and method which utilize an external optical member with a Gaussian intensity profile promoting cavity or surface for improving or enhancing the overall beam quality of broad-area semiconductor lasers.
2. The Relevant Technology
Lasers are widely used in various technologies such as telecommunications, medical treatments, remote sensing, printing, etc. The need for higher power lasers has increased over the years in some technology areas. Conventionally, in order to provide a laser with increased power, it was necessary to increase the size of the laser, resulting in a substantial cost increase. Thus, an expensive laser that filled an entire room could provide a desired power level, but was infeasible for many applications.
Semiconductor lasers were developed to meet the need of smaller size for lasers in certain applications. The compact size of semiconductor lasers is advantageous in applications such as optical fiber communications. Most high power semiconductor lasers are made in the so-called "stripe geometry" and are of the "broad-area" type, where the term "area" refers to the active area of the junction from where the laser radiation is emitted. The width of the active area is typically in the range of about 50 .mu.m to about 500 .mu.m and the thickness is typically on the order of 1 .mu.m or less.
It is desirable to make the active layer thickness as thin as possible to achieve lower threshold current density. By making the active layer thinner, however, the aperture from which the laser radiation is emitted is also smaller, which makes the beam subject to a large divergence due to aperture diffraction effects. In order to achieve maximum power in the beam, a larger active layer is desirable, as the peak power of a semiconductor laser is directly proportional to the active area of the emitting facet of the device.
The achievement of high power, however, results in decreased laser beam quality. For a broad-area gain region, lateral confinement becomes so weak that the lateral mode profile of the beam can break into multiple filaments. In particular, the beam is broadly divergent, which results in a power distribution that is not focused into one spot or point. Instead the power distribution is spread over a larger area and into separate filaments. This problem, commonly referred to as filamentation, has made it extremely difficult to develop broad-area semiconductor lasers that are efficient and capable of a high level of output power.
Various approaches have been developed to alleviate the beam filamentation problem in broad-area semiconductor lasers. A conventional technique implements an external reflective cavity to control and optimize laser mode profiles through proper cavity design. For example, external cavity configurations have been used employing a planar, cylindrical, or spherical mirror together with a spherical or cylindrical lens or a system of such lenses, to compensate for the filamentation problem inherent in broad-area lasers. Such techniques are disclosed in U.S. Pat. No. 5,050,179 to Mooradian.
The use of such conventional lens and mirror systems with semiconductor lasers presents difficulties and is costly to implement effectively. The divergent beams can be difficult to align using the lens and mirror system. Further, the accurate placement and size of each lens complicates the design and typically results in a reduction in the efficiency of the system.
Accordingly, there is a need for improved semiconductor laser systems that overcome or avoid the above problems and difficulties.